Orthodontic bracket assembly

ABSTRACT

An orthodontic bracket assembly for bonding to the surface of a tooth is disclosed. The assembly includes a pad and an archwire bracket. A plurality of cavities are provided in the tooth abutting surface of the pad. The walls of the cavities are provided with an irregular surface texture to enhance the bonding between the tooth and the orthodontic bracket. In the preferred embodiment the bracket is cast. The external surface of the bracket is smoothed in a tumbling operation in which the particle size of the abrasive is chosen so that the cavity walls are not subjected to treatment but are left in their irregular, as-cast state. Also disclosed is a method for preparing the mold to produce the cavities in the tooth-abutting surface of the pad.

BACKGROUND OF THE INVENTION

This invention relates to orthodontic appliances, particularly toorthodontic brackets of the type which are bonded to the externalsurface of the user's teeth.

Orthodontic brackets which are fastened to the surface of the wearer'steeth by being bonded directly thereto have become quite common. Thesebrackets are typically attached to the outer surface of a pad having anopposite surface configured to generally conform to the shape of thewearer's tooth. This inner, or tooth abutting, surface of the bracketpad is adapted to be attached to the wearer's tooth, such as byadhesive.

One problem with using these bonded orthodontic brackets is a propensityof the pads to break away from the tooth. Among the ways to increase thebonding strength between the adhesive and the bracket is to form thepads with holes through their thickness. This allows adhesive to flowfrom the tooth abutting surface of the bracket pad, through the hole andon to the outer surface of the bracket pad. In this manner, the adhesivewill form a strong mechanical bond between the bracket pad and tooth.However, it has been found that the adhesive on the outer surface of thepad can be worn away during brushing and thus lose much of its grippingpower. Further, the desire for an intimate bond between the bracket padand the adhesive is not addressed in this approach.

Another approach for increasing the bonding strength between theadhesive and the bracket is to sandblast the tooth abutting surface ofthe bracket pad and/or chemically etch the surface to produce bulbouspores on the tooth abutting pad surface. Either or both of theseproceedures can be relatively expensive and also fail to significantlyincrease the total surface area of the pad to which the adhesive can bebonded.

A third manner of increasing the strength of the bond is to incorporatea wire mesh on the bracket pad and the tooth abutting surface. Thisagain is relatively expensive and can result in a bracket which isthicker than desirable.

Examples of orthodontic brackets incorporating the features describedabove can be found in U.S. Pat. Nos. 3,765,091; 4,068,379; 4,100,678;and 4,165,561.

SUMMARY OF THE INVENTION

A cast orthodontic bracket assembly for bonding to the surface of atooth and having a plurality of cavities within the tooth abuttingsurface of the bracket is disclosed. The walls of the cavities areprovided with irregularities to enhance the bonding strength between thebracket and the tooth. It is preferred that the cavity walls be providedwith surface irregularities from the process of casting the bracket. Bythe proper choice of cavity size and finishing techniques the cavitywalls of a cast bracket can be retained in substantially the as-castcondition while the remaining surfaces of the bracket assembly arefinished to the desired smoothness. As a result, the cavities providesurfaces to which conventional adhesives can be applied that produceheretofor unattainable bonding characteristics.

While prior art orthodontic brackets have been formed with cavities inthe tooth abutting surface, such brackets have met with littlecommercial success because of the limited surface available for adhesiveattachment. It has now been found that by casting the bracket withcavities in the tooth abutting surface, the natural castingirregularities imparted to the surface of the cavities producesubstantially greater area for adhesive contact. Greater bondingstrength results. The casting irregularities of the cavity walls alsoaid the mechanical bonding strength of the adhesive/bracket cavity wallbond.

Also disclosed is a novel method of forming the cavities on thetooth-abutting surface of the bracket pad. In casting the brackets,molds are made which are the inverse of the finished bracket. The toothabutting surface generally has a critical, compoundly curved surfaceshape for the best bonding to the tooth surface. According to thedisclosed method for forming the mold, the cavities are formed bycutting two perpendicular series of parallel cuts in the mold. Since thebase of these cuts form the tooth abutting surface, the depth of thecuts must be carefully controlled.

To form the cuts having the desired variable depths, the molds aremounted so that they can move along an arc corresponding to the desiredshape of the tooth-abutting surface. After a cut is made, the mold isindexed about a first axis in space, the radius from the axis to thebracket corresponding to the instantaneous radius of curvature of thedesired tooth-abutting surface. A new cut is made parallel to theprevious cut and along the desired arc to achieve the desired depth ofcut. Generally the arcs of the cuts are circular so that the cuts can bemade by moving the bracket (or the cutting tool) along a circular arc;however, they need not be circular. After the entire series of parallelcuts is made, the mold is rotated 90° and a second series of parallelcuts is made about a second set of axes.

The resulting cavities in the bracket, formed when the discribed mold isused in an investment casting process to produce the bracket, areslightly larger at the bottom than at the top due to the rotation of themold about the first axis to produce the parallel cuts. The bondingstrength between the adhesive and the bracket pad is thereforeincreased.

The entire orthodontic bracket is preferably cast as a single elementwith a plurality of cavities preformed into the tooth abutting surfaceof the bracket pad. Provision of an orthodontic bracket cast as anintegral unit provides a small, strong and accurately reproduced bracketat low cost.

The cast orthodontic bracket is placed in a tumbler having an abrasivematerial to remove sharp edges and flashing on the casting as well as tosmooth the surface of the bracket. Smoothing the relatively rough castsurface is desirable for aesthetic reasons and for the comfort of thewearer. The particle size of the abrasive is chosen so that the surfacesof each cavity, i.e., the cavity walls, are not abraded smooth but areleft in their roughened, as-cast state. After tumbling to smooth theexterior surface, the orthodontic bracket can be placed in a secondabrasive medium for polishing. During polishing the interior surface ofthe cavities remains substantially unaffected by the polishing abrasive,while the external surface is further smoothed to the desired degree ofluster.

The disclosed method for smoothing and polishing the exterior surface ofthe bracket is inexpensive and very effective. It has been found that nospecial precautions, other than judiciously choosing the tumblingabrasive, need be taken with regard to the cavities. The competinginterests of a smooth exterior surface versus a roughened bondingsurface are thus effectively balanced in a simple and inexpensivemanner.

Other features and advantages of the present invention will appear fromthe following description in which the preferred embodiment has been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of the orthodontic bracket shownbonded to the exterior surface of a tooth, the tooth shown in dottedlines.

FIG. 2 is an enlarged perspective view of the orthodontic bracketillustrating the plurality of cavities formed within the tooth abuttingsurface of the bracket pad.

FIG. 3 is a photograph taken through an electron microscope of the toothabutting surface in its as-cast condition.

FIG. 4 is a photograph taken through an electron microscope of the toothabutting surface after tumbling showing the cavity walls still in theiras-cast condition.

FIG. 5 is a perspective view of a portion of the mold illustrating themanner of making the cuts in the mold to produce the cavities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the figures, the orthodontic bracket 2 of the presentinvention includes generally a curved rectangular bracket pad 4 and apair of archwire brackets 6 extending from the outer surface 8 of thebracket pad.

The bracket pad has an inner tooth abutting surface 10 opposite outersurface 8. The tooth abutting surface has a plurality of cavities 12formed therein and extending into the interior of the bracket pad. Thecavities, shown best in FIGS. 2 through 4, form a waffle-like patternalong surface 10. The cavities are defined by four cavity sides 14 and acavity bottom 16. Collectively the cavity sides and cavity bottom aretermed the cavity walls.

Archwire brackets 6 are of standard configuration and include a pair ofL-shaped members 18 defining an archwire groove 20 therebetween. It ispreferred to cast the orthodontic bracket as a single piece of bothstrength and economy.

The cavities are preferably formed by creating a mold wherein groovesare cut in the bracket pad portion of the mold. Since the mold is theinverse of the bracket, a cavity in the mold corresponds to a solidportion on the bracket and visa versa. Turning to FIG. 5, the method ofproducing the waffle-like tooth abutting surface 10 of bracket pad 4will be described.

FIG. 5 schematically illustrates the formation of the proper surfaceshape of mold portion M. Surface 30, which corresponds to tooth abuttingsurface 10 of the bracket pad, is formed by first making four cuts32-35. Cut 32 is made in the following manner. Mold portion M is indexedon an arc about axis A to positione a cutter C at one end of theproposed cut. Portion M is rotated about axis B to make cut 32 by thecutter. Mold portion M is indexed about axis A to align the cutter withproposed cut 33 and the procedure is repeated. After cuts 32-35 aremade, mold portion M is rotated 90° so that transverse cuts 40-50 can bemade in the manner indicated above. The transverse cuts are generallymade about new axes A' and B', not shown, so that surface 30 willproduce a tooth abutting surface 10 of the bracket having an appropriatecompound contour for the best bonding to the tooth.

By making the cuts as described above, projections 48, which form thecavities in the molded bracket, are slightly smaller at their basesadjacent surface 30 than at their top surfaces 52. Therefore, theresulting cavities are slightly narrower at their tops than at theirbottoms to enhance the strength of the bond.

The above described method of producing the mold produces cavity sides14 which are generally perpendicular to tooth abutting surface 10 andfurther produces surface irregularities on sides 14 which areperpendicular to sides 14. These irregularities produced by the novelmold production technique therefore greatly enhance the bonding strengthbetween the adhesive and cavity sides 14 compared with cavities havingsurface irregularities not so directed.

In lieu of moving the mold portion M, the cutter could be indexed androtated about the axes. If desired the mold portion can be indexed aboutaxis A and the cutter can be moved about axis B. Also, the movement ofthe cutter or mold portion while making the various cuts can be alongpaths which are not circular depending upon the shape of the toothabutting surface desired. The mold may also be rotated an angle otherthan 90° so that the cavities produced will be diamond-shaped ratherthan rectangular.

The cavity walls are left in their roughened, as-cast state. This isaccomplished by first tumbling the cast bracket in an abrasive medium,the particles of which are of sufficient size so that the cavity wallsremain substantially unaffected. However, the remainder of the exteriorsurface of the bracket, that is the exterior surface other than thecavity walls, is smoothed by the tumbling in the abrasive medium. Oneparticular tumbling method uses triangular shaped rocks in a water bath.The rocks are approximately ten times the size of the brackets. Thebrackets, rocks and water are placed in a vibrator to constantly agitatethe contents to smooth the exterior surfaces.

Polishing the balance of the smoothed surface to produce the desiredluster can be accomplished by tumbling in a polishing medium. This canbe done by using stainless steel shot in a water bath. The shot is ofdifferent sizes, the smallest typically a number of times as large as abracket. The shot, water and brackets are agitated in a vibrator untilthe exterior surfaces achieve the desired luster. Of course otherappropriate tumbling methods, media and sizes of media can be used forthe preliminary or finish tumbling steps.

FIGS. 3 and 4 show in a striking manner the effects of the abovedescribed structure and process. While the upper areas of the toothabutting surface of FIG. 4 are relatively smooth, the cavity wallseffectively retain their as-cast character.

Modification and variation may be made to the preferred embodimentwithout departing from what is regarded as the subject of the inventionas defined in the following claims. For example, the cavities may beround and dispersed randomly over the tooth abutting surface of thebracket pad. Also, some or all of the cavities may extend entirelythrough the bracket pad.

What is claimed is:
 1. A cast orthodontic bracket for bonding to thesurface of a tooth comprising:an archwire bracket portion adapted toreceive an archwire; a pad portion attached to said archwire portion andhaving a tooth abutting surface opposite said archwire portion; saidtooth abutting surface having a plurality of cavities formed therein,said cavities having a top opening, a circumferential sidewall, and abottom, said sidewall extending downwardly and outwardly toward saidbottom and away from a central portion of said cavity so said cavity issmaller in all directions at said top than at said bottom; and saidcavities having irregular surfaces, a substantial portion of theremainder of said tooth abutting surface and of the surface of saidbracket being a relatively smooth surfaces produced when said bracket istumbled in an abrasive medium adapted so as not to smooth said irregularsurfaces of said cavities.
 2. The orthodontic bracket of claim 1 whereinsaid cavities have four sides, each said side extending from said topand away from the opposing side.
 3. An improved orthodontic bracket ofthe type including a pad having a surface adapted to be bonded to theexterior surface of a tooth, the pad having a tooth abutting surfacewith a plurality of cavities, said cavities defined by cavity walls,said bracket produced by a process comprising the followingsteps:casting said bracket as a unitary piece with the surface of saidbracket having a cast irregular surface character; providing an abrasivemedium; and tumbling said cast bracket in the abrasive medium, theparticles of the abrasive medium adapted so as not to smooth over thewalls of said cavities but to smooth a substantial portion of theremainder of said tooth abutting surface and of said bracket surface. 4.The bracket produced by the process of claim 3 further comprising thestep of polishing said smoothed bracket surface of said tumbled bracket.5. The bracket produced by the process of claim 4 wherein the polishingstep includes the step of polish tumbling said bracket.
 6. A method forproducing an orthodontic bracket of the type having a pad with cavitiesformed in a tooth abutting surface of the pad for bonding to a tooth,the cavities defined by cavity walls, the method comprising:forming abracket mold with mold surfaces having a roughened surface character;casting the bracket in the bracket mold, the surfaces of the brackethaving a roughened surface character from the mold surfaces; providingan abrasive tumbling medium including abrasive particles; tumbling thebracket in the abrasive medium; and said providing step including thestep of sizing the particles of the abrasive medium so the cavity wallsare substantially unaffected by the abrasive medium and the remainder ofthe bracket surfaces are substantially smoothed over by the abrasivemedium.
 7. The bracket producing method of claim 6 further comprisingthe step of polish tumbling the bracket in a polish medium after thetumbling step.
 8. The bracket producing method of claim 6 wherein saidforming step includes the step of forming a bracket mold adapted to makea one-piece cast bracket.